As is known, integrated circuits and other active electronic devices or components are made on "chips" of semiconductor material having a surface area of a few mm.sup.2, and require, for their connection to an external electrical circuit, suitable supporting, containment and electrical interconnection structures. A typical structure (package) suitable for the purpose includes a plastic body which encloses a chip which is soldered to an area of a metal supporting element of a metal structure formed by punching from a thin sheet and is connected, through thin metal wires soldered to suitable metallized areas (pads) provided on its surface, to corresponding electrical conductors which emerge from the plastic body and which also form part of the metal structure.
In the case of integrated power circuits, in other words devices intended to operate with high currents and, therefore, subject to considerable heating, these structures also comprise a metal plate through which the chip, which is fixed to it, can transfer to the exterior the heat produced during its operation.
The principal stages of the manufacture of the latter structures will now be described. The plate is formed by punching from a metal sheet, made for example of copper or copper alloys, possibly nickel-plated, possibly together with other identical plates which remain joined together by portions of the initial sheet which are designed to be removed by punching in a subsequent stage of processing. The chip is fixed on the metal plate by soldering with a low-melting-point alloy, for example a lead-tin alloy, or by gluing with a suitable adhesive, for example an epoxy adhesive. A set of metal strips, formed by punching from a thin sheet but still joined together by connecting portions and designed to become the terminal conductors of the device, are fixed to the plate, with at least part of the metal strips insulated electrically from the plate. Thin wires, usually made from gold, are welded at one end to the metallized areas of the chip with a low-melting-point alloy and at the other end to the ends of the metal strips, using a process, called "thermosonic", in which heat and ultrasound are applied simultaneously. The structure formed in this way, together with other identical structures having their corresponding plates and corresponding sets of interconnected metal strips, is then placed in a suitable mould, into which a plastic material in the liquid state, for example a thermosetting epoxy resin, is injected. After the polymerization of the resin, a multiplicity of structures is obtained, each comprising a solid plastic body which incorporates the elements described above with the exception of one face of the metal plate and part of the metal strips, in other words the terminal conductors of the device, and the interconnecting portions between them. These portions, together with the interconnecting portions between the plates, are then cut off and in this way the finished electronic devices are obtained.
A typical problem with the structures (packages) described above, and in general of any structure consisting of a plastic body and metal elements formed from sheets having a high level of surface finish, in other words a low roughness (for example, R.sub.a.ltoreq.0.1 .mu.m in the case of nickel, R.sub.a.ltoreq.0.4 .mu.m in the case of copper and copper alloys), is the low reliability, due to insufficient adhesion of the plastic body to the metal parts. It has been found that many of the failures of these devices are due to the ingress of moisture into the body through interstices between the metal terminals and the plastic body which are formed as a result of the cutting force, as a result of the difference between the coefficients of thermal expansion of the metal and the plastic, which is manifested between the metal and plastic in the cooling phase after the molding operation and during the normal thermal cycles of operation.
To resolve this problem at least partially, in other words to provide greater reliability and a more moisture-resistant seal, a known method is to treat the metal surfaces in such a way as to increase their roughness, for example by subjecting them to compression with molds having surfaces of predetermined roughness, or by subjecting them to sandblasting.
The technique based on molding is rather expensive, since it requires the preparation of individual molds and frequent maintenance operations to restore the desired degree of roughness of the mold, and the technique based on sandblasting is not suitable for use when the metal elements to be treated are of very small size, as in the case of lead frames for integrated circuits with a large number of pins. Furthermore, when a selective treatment of the metal surfaces is desired (as is often required in the case of frames with areas which must retain the original degree of finish to enable a chip of semiconductor material to be soldered to them), both of these known techniques require the use of masks: the first during the preparation and maintenance of the mold, and the second during the actual treatment. This further complicates the process.